1. Field of the Invention
This invention relates generally to a method for controlling the output current of a fuel cell stack and, more particularly, to a method for immediately reducing the output current of a fuel cell stack if either the minimum cell voltage or the stack voltage drops to a predetermined voltage set-point, and then increasing the allowed current in a controlled manner if the minimum cell voltage or the stack voltage increases above the set-point.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
As is well understood in the art, if a minimum cell voltage or the overall stack voltage falls below a predetermined value, then cell voltage reversal becomes a possibility that may lead to a rapid reduction in the catalyst carbon support in the MEA, ultimately lowering the cell voltage and overall system durability and reliability. For example, for a minimum cell below 300 mA, it is desirable to reduce the current output from the stack because the low performing cell could generate a significant amount of heat, and if the voltage output of the cell goes below zero, it will begin to corrode the carbon in the MEAs.
Typically, it is a significant challenge to maintain a minimum stack voltage and at the same time allow for very fast up-transients and current draw from the stack. For those cases where the current draw from the stack must be reduced to avoid too low of a stack voltage, it has been a challenge to know how to reduce the current quickly and smoothly to avoid oscillations or more loss in power than is necessary. Further, it has been a challenge to know when to start adding allowed current back when the stack recovers. If the added current subsequently sends the stack voltage low again, the process to avoid oscillation is yet another challenge.
There are known techniques for reducing allowed stack current as the stack voltage and/or minimum cell voltage falls below a threshold. One known technique uses a modeled voltage/current curve and limits current based purely on a predicted voltage/current slope of the curve. The problem with this implementation is the slope often intervenes in a situation it does not need to, typically too harshly, thereby limiting transient rates. It has also been known to not intervene in situations where it should have. Another alternative can be to use a standard proportional-integral (PI) controller without a bias where there is an error generated that is amplified by the P and I gains to reduce current. The problem with this implementation without a bias is that if the feedback voltage goes below the threshold at a low current, then there is a period of time where the P and I gains are reducing current from the maximum system current, but not reducing the actual system current. As a result, the current can rise while it should be falling and valuable intervention time is lost. In an effort to increase the response of this implementation, it is tempting to increase the P and I gains. However it is then very easy to trigger heavy oscillations and/or overly aggressive reductions. The stack voltage threshold is selected to be some value above a true minimum stack voltage where high voltage components in the system will shut down to protect the system.